1) Field of the Invention
The present invention relates to a method and an apparatus for calibration of a camera system, and a method of manufacturing the camera system.
2) Description of the Related Art
Recently, a camera system that recognizes information, for example, position information, of an object in an image taken by a camera has been developed.
For instance, the technology that recognizes an obstacle from a picture taken by two or more cameras is proposed in Japanese Patent Application Laid-open Publication No. 2001-243456.
Such a camera system is sometimes installed on the vehicles. A camera, which may be installed inside or outside the vehicle, captures an image of a forward area of the vehicle. It is recognized from the image whether an obstacle (e.g. other vehicles) is present in front of the vehicle. If an obstacle is present, a distance between the obstacle and the vehicle is calculated. This camera system is calibrated immediately prior to its shipment. The calibration process includes calibration of the installation position of the camera as well as setting of various parameters that are required for taking a picture.
The calibration is performed according to the following procedure: 1) fix a camera at a predetermined position so that the camera covers a region in which two or more marks are placed; 2) take a picture of a region and obtain position information of the marks (hereinafter, “picture mark information”) in the picture; and 3) perform calibration process based on a difference between the picture mark information and actual position information of the marks (hereinafter, “actual mark information”), which is know in advance. The calibration can be performed more accurately if the region covered by the camera is large and when there are lot of marks in the region. In other words, for better accuracy, a large space (e.g. distances of about several tens to a hundred meters from the camera) and a lot of marks are required.
Other approach is to use the Planar Projection Stereopsis. A camera system that employs the planar projection stereopsis is disclosed in, for example, Japanese Patent Application Laid-open Publication No. 2001-243456. In this camera system, since it is necessary to obtain parameters for plane projection, its calibration also requires information that indicates how the plane is projected on the picture. In other words, for better accuracy, a large plane and a lot of marks are required.
Thus, the conventional camera systems give better accuracy only if a large space is available. This requirement makes it impossible to perform the calibration inside the factory where the camera system is manufactured. As a result, to perform the calibration, the camera system is installed on a vehicle and the vehicle is carried to a place such as a road, which is outside the factory, where a lot of marks are placed. However, this makes the process cumbersome.